Magnetic memory devices (HDDs) mainly used in computers to record and reproduce information are recently beginning to be used in various applications because they have large capacities, inexpensiveness, high data access speeds, a high data retaining reliability, and the like, and they are now used in various fields such as household video decks, audio apparatuses, and automobile navigation systems. As the range of applications of the HDDs extends, demands for large storage capacities increase, and high-density HDDs are more and more extensively developed in recent years.
As a magnetic recording method of presently commercially available HDDs, a perpendicular magnetic recording method is recently rapidly finding widespread use as a technique replacing the conventional in-plane magnetic recording method. In the perpendicular magnetic recording method, magnetic crystal grains forming a perpendicular magnetic recording layer for recording information have the axis of easy magnetization in a direction perpendicular to a substrate.
To increase the recording density of the perpendicular magnetic recording medium, noise must be reduced while a high thermal stability is maintained. A method generally used as a noise reducing method is to reduce the magnetic interaction between the magnetic crystal grains in the recording layer by magnetically isolating the grains in the film surface, and decrease the size of the grains themselves at the same time. A practical example of the generally used method is a method of adding SiO2 or the like to the recording layer, thereby forming a perpendicular magnetic recording layer having a so-called granular structure in which each magnetic crystal grain is surrounded by a grain boundary region mainly containing the additive. If noise is reduced by this method, however, it is inevitably necessary to increase the magnetic anisotropic energy (Ku) of the magnetic crystal grains, as the magnetization reversal volume reduces, in order to ensure a high thermal stability. If the magnetic anisotropic energy of the magnetic crystal grains is increased, however, a saturation magnetic field Hs and coercive force Hc also increase. Since this increases a recording magnetic field necessary for magnetization reversal for data write as well, the writability of a recording head decreases. As a consequence, the recording/reproduction characteristics deteriorate.
To solve this problem, a patterned medium in which magnetic dots are formed by micropatterning a perpendicular magnetic recording layer and are magnetically isolated from each other is being studied. In the patterned medium, the magnetic crystal grains in the perpendicular magnetic recording layer need only be magnetically isolated not for every crystal grain but for every magnetic dot having a size corresponding to one bit and containing a few to a few ten crystal grains. This makes the magnetization reversal volume larger than that of the granular structure. Accordingly, the Ku value required to assure a high thermal stability can be decreased. This makes it possible to suppress the increases in Hs and Hc, and suppress a magnetic field (switching field) necessary for magnetization reversal.
On the other hand, in the patterned medium, the switching field distribution (SFD) of each magnetic dot must be minimized in order to allow a designated magnetic dot to reliably reverse magnetization with respect to a recording magnetic field having a given preset intensity, and prevent magnetization reversal of adjacent dots. One big cause of the SFD is the influence of the magnetostatic interaction between dots. That is, the magnitude of a demagnetizing field generated in a given dot from surrounding dots changes in accordance with the difference between recording magnetization patterns. Consequently, an effective switching field changes in accordance with a recording pattern. Since the magnetostatic interaction between dots increases as the dot pitch decreases, the SFD increases as the density increases.